The numbers of cases and cause-specific deaths refer to data recorded in National Notifiable Disease Reporting System in China, which includes both clinically-diagnosed cases and laboratory-confirmed cases. Only reported cases of the 31 provincial-level administrative divisions in Chinese mainland are included in the table, whereas data of Hong Kong Special Administrative Region, Macau Special Administrative Region, and Taiwan, China are not included. Monthly statistics are calculated without annual verification which is usually conducted in February of the next year for de-duplication and verification of reported cases in annual statistics. Therefore, 12-month cases could not be added together directly to calculate the cumulative cases because the individual information might be verified via National Notifiable Disease Reporting System according to information verification or field investigations by local CDCs.

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Seasonal Patterns: Examining the data before July 2023, certain seasonal patterns in case numbers can be observed. Typically, there is a peak in cases during the winter months (December to February) and a decline during the summer months (June to August). This indicates a seasonal pattern in mainland China, with increased disease transmission during colder months and decreased transmission during warmer months.
Peak and Trough Periods: Based on the data, the peak periods for cases occur consistently during the winter months, particularly in December and January. These months consistently exhibit higher case numbers compared to other months. Conversely, trough periods, characterized by relatively lower case numbers, occur consistently during the summer months, particularly from June to August.
Overall Trends: Over the years leading up to July 2023, there is an overall increasing trend in case numbers in mainland China. From 2010 to 2019, there is a relatively gradual rise in cases, with some year-to-year variations. However, starting in 2020, there is a significant increase in cases, potentially due to the impact of the COVID-19 pandemic. The highest number of cases is observed in March 2023, with a steep rise evident from 2021 onward.
Discussion: The observed seasonal patterns suggest that disease transmission in mainland China follows a seasonal trend, with higher transmission during the winter and lower transmission during the summer. This pattern is consistent with that seen in numerous respiratory diseases, including viral pandemics like COVID-19.
The peak and trough periods reveal the times when transmission is highest and lowest. Peak periods during the winter months indicate heightened vulnerability and increased risk of transmission, while trough periods during the summer months suggest reduced transmission and lower risk.
However, it is important to note that the overall increase in case numbers before July 2023, particularly from 2020 onward, may be influenced by factors beyond seasonal fluctuations. The impact of the COVID-19 pandemic, in particular, may significantly contribute to the observed upward trend.
These findings underscore the significance of comprehending seasonal patterns and overall trends in disease transmission for effective public health planning and control measures. By identifying peak periods and anticipating seasonal variations, public health interventions can be tailored to mitigate the impact of the disease.